If you have downloaded a ".dockzip" file, simply double-click on it.
If you have downloaded a zipped folder (.zip) then extract the folder into one of these two places:
"C:\Program Files (x86)\Stardock\ObjectDock\Backgrounds" or "Documents\Stardock\ObjectDock Library\Backgrounds"
Once you have done that, open your OD config, go to the "Appearance" tab, and select your new background from the drop-down list.
